Gamma-prime (γ′) precipitation-strengthened nickel-base superalloys with a combined fraction of Al and Ti of about more than 5 wt.-% are known to be very difficult to weld because of their micro-crack sensitiveness.
In the document: B. Geddes, H. Leon, X. Huang: Superalloys, Alloying and performance, ASM International, 2010, page 71-72, the authors describe a weldability line for superalloys approximately as [two times Al concentration (wt.-%)+Ti concentration (wt. %)]<6.0, this means that Ni base superalloys with more than 6 wt.-% of [2 Al (wt.-%)+Ti (wt.-%)] are defined as difficult to weld materials. Solidification and grain boundary liquation cracking occurs during the welding process, whereas post-weld heat treatments often lead to strain age cracking in gamma-prime Ni3(Al,Ti) precipitate strengthened alloys. Therefore, mainly solid-solution strengthened (e.g. IN625) or gamma-prime strengthened nickel-base superalloys with a low amount of Al and Ti (e.g. In718) can be processed by SLM up to the present day.
In a common approach to process difficult to weld gamma-prime precipitation-strengthened nickel-base superalloys, a powder bed is heated to an elevated temperature to reduce residual stresses resulting from the welding process. But, before the finished parts can be removed from the powder bed, it has to be cooled down to ambient temperature. Due to the low heat conductivity of powder beds, the heating up and cooling down of the powder bed requires a lot of time resulting in a significant decrease in productivity of the SLM process. Furthermore expensive heating equipment and isolation as well as adaptation of the process chamber are needed.
The following literature is related to these technologies and problems:    (1) Kelbassa, I., et al. Manufacture and repair of aero engine components using laser technology. in Proceedings of the 3rd Pacific International Conference on Application of Lasers and Optics. 2008;    (2) Mumtaz, K. and N. Hopkinson, Top surface and side roughness of Inconel 625 parts processed using selective laser melting. Rapid Prototyping Journal, 2009. 15(2): p. 96-103;    (3) Mumtaz, K. and N. Hopkinson, Laser melting functionally graded composition of Waspaloy® and Zirconia powders. Journal of Materials Science, 2007. 42(18): p. 7647-7656;    (4) Mumtaz, K. A., P. Erasenthiran, and N. Hopkinson, High density selective laser melting of Waspaloy®. Journal of Materials Processing Technology, 2008. 195(1-3): p. 77-87; and    (5) Sehrt, J. T. and G. Witt, Entwicklung einer Verfahrenssystematik bei der Qualifizierung neuer Werkstoffe für das Strahlschmelzverfahren. 2010. Publication of trials to process difficult to weld gamma-prime precipitation-strengthened ni-base superalloys.
Furthermore, document U.S. Pat. No. 6,215,093 B1 discloses a method for manufacturing a moulded body, in accordance with three-dimensional CAD data of a model of a moulded body, by depositing layers of a metallic material in powder form. Several layers of powder are successively deposited one on top of the other, whereby each layer of powder is heated to a specific temperature by means of a focused laser beam applied to a given area corresponding to a selected cross-sectional area of the model of the moulded body, before deposition of the next layer. The laser beam is guided over each layer of powder in accordance with the CAD cross-sectional data of the selected cross-sectional area of the model in such a way that each layer of powder is fixed to the layer below it. Especially, the metallic material in powder form is applied in the form of a metallic powder free of binders and fluxing agents, that it is heated by the laser beam to melting temperature, that the energy of the laser beam is chosen in such a way that the layer of metallic powder is fully molten throughout at the point of impact of said laser beam, that the laser beam is guided across the specified area of powder in several runs in such a way that each run of the laser beam partly overlaps the preceding run, and that a protective gas atmosphere is maintained above the interaction zone of the laser beam and the metallic powder.
Document DE 10 10 4732 C1 teaches a device for selective laser melting of metallic materials comprising a heating plate arranged on a platform with side walls. The heating plate is structured so that an insulating layer is thermally insulated from the platform so that temperatures of 500 deg C. can be achieved during the operation. Preferably, the heating plate is formed as a substrate plate and has integrated heating wires. An induction unit is provided for inductively heating the heating plate.
Document U.S. Pat. No. 6,621,039 B2 discloses a computer-controlled apparatus and method for producing metallic parts by laser melting selected regions of layers of metal powder at a target area. The system includes devices for preheating and maintaining a relatively high temperature, e.g. 400° C., of the metal powder so as to join the metal powder together with relatively low laser power, e.g. a 200 W CO2 laser. The metal powder is preheated at either a dispensing cylinder or the target area through thermal conduction and/or is also heated by a heating plate positioned above the platform through radiation.